tcp_diag and tp->ca_ops turn

I have been working now to remove the last TCP specific bits in net/ipv4/tcp_diag.c, and to accomplish this the TCP CA avoidance infrastructure will be moved to struct inet_connection_sock, antecipating something I planned to do anyway, i.e. to generalise this infrastructure to use it in DCCP and who knows, even turn it into a generic infrastructure for use by other protocols or even other unrelated kernel areas (whee, here I’m possibly going over the top, time will tell).